transmission question 350 Turbo or 700R

[Yooper truck]

My 71 Chevy Pickup has a 350 Turbo transmission. It slips a little when it is cold, but otherwise is fine. I am planning on changing the fluid and filter to see if that cures the slipping. Mileage on the transmission is unknown.

I have 700R transmission in an 84 suburban I am junking.

If I was to have either transmission rebuilt, which would be the better choice? (I am looking for a cruiser, not a hot rod)

Would I get any improvement in gas mileage with the 700R having the overdrive?

If it is better to keep the 350 Turbo, then is the 700R worth taking out to sell separately while scrapping the rest of the vehicle?

Thanks for any advice.... I have been mulling this over and need input from others.